The SERS (surface enhanced Raman spectroscopy) effect arises from an electromagnetic enhancement of optical processes near noble metal surfaces. The magnitude of the enhancement varies from metal to metal and depends on the surface preparation. Silver shows the largest enhancement, on the order of 106 to 107 increased Raman signal strength.
Previous SERS methods only work well for a limited class of analyte molecules. SERS sensors have been made for organic compounds, metal ions, pH, and CO2 detection with good sensitivity and selectivity. A need exists for methods to produce sensitive and selective Raman sensors for use with other types of analytes.
A problem with SERS technology has been the lack of reproducibility of the surfaces used for surface enhancement. In particular, the reproducible production of surfaces uniformly coated with metal or metal alloy nanoparticles has been difficult to achieve. The synthesis of metal colloids can be unpredictable, resulting in nanoparticles of varying sizes that make SERS detection unreliable. In addition, surface chemistry has been limiting with respect to production of surfaces containing multiple Raman-active metals. The generation of Raman surfaces with even low levels of periodicity, allowing fine-tuning of surfaces for detecting different analytes, has been expensive to scale-up.
Recent advances in electron-beam, ion-beam, and scanning probe lithography techniques have pushed the minimum size of microfabricated structures below the 100 nm scale. However, generation of homogeneous nanoparticle layers has not progressed equivalently. A reliable and reproducible method is needed to generate uniform nanoparticle surfaces for use in Raman spectroscopy. Such uniform nanoparticle surfaces could be used in combination with microfabricated structures to provide the surfaces for the sensitive and reliable detection of a wide range of analytes.